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Moholaser
Now in the "Science: Geology" category! | |
Back in the 1960s, a project was begun to drill through the
Earth's crust to the Mohoroviç discontinuity, marking the
boundary between the crust and the mantle.
Had the project - known as Project Mohole - succeeded, it
would have been immensely valuable. Its value, of course,
would have lain
in being able to say "we drilled through the
Earth's crust!".
Sadly, Project Mohole fizzled out (though it did give birth to
the science of dynamic positioning for drilling rigs). Other
very deep holes (notably the Kola Superdeep Borehole) have
been drilled, but none has got more than half way through
the
crust.
But now we have lasers and, as is well known, lasers make
everything better. Proposed, therefore, is Project
Moholaser.
A high-power pulsed laser is simply pointed downwards, and
is
roofed over by a dome from which air can be evacuated.
Each
laser pulse will melt and vaporise a few millimetres of rock,
which will be drawn out of the hole by the vacuum. With a
few pulses per second (easily attainable), about a month
should be enough to reach the mantle.
The depth attainable in this way is limited only by the
Werther's Limit - which is slightly below the Mohoroviç
discontinuity, and is the point at which rock becomes
sufficiently toffee-like to flow more quickly than it can be
vaporized by the laser.
Lava Wells
Lava_20wells
Prior Art [8th of 7, Oct 18 2018]
http://home.earthli...et/~jimlux/lava.htm
[hippo, Oct 18 2018]
http://www.madsci.o...015040902.Es.r.html
[hippo, Oct 22 2018]
[link]
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Cool, but wouldn't this just promote a man-made volcano to form? |
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and... so where do we set this thing up? |
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If the Earth's crust is 30km thick and you want to melt a hole of cross-sectional area 0.5m^2, then there's about 15000m^3 of rock to melt. If rock is about 3000kg/m^2, then this is 45,000,000kg of rock. Rock requires about 900kJ/kg to melt it (see link), so this is 40.5x10^9 kJ, or about 40TJ.
There are 2.5m seconds in a month, so this requires a continuous 16.2MW laser, assuming perfect efficiency at extracting the molten rock and no heat conduction to surrounding rock. |
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So, a little hobbyist-type laser - all pretty practical, then. Heat loss shouldn't be an issue due to the low thermal conductivity of rock, and besides as the hole gets deeper the surrounding strata get much hotter tending to help the process. |
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//wouldn't this just promote a man-made volcano to form?// I once proposed the creation of a small volcano for a public art installation. It was rejected, so it's still awaiting funding to proceed. |
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That's probably related to the difference between the human perception of "small" and the geological reality. |
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Mud volcanoes can be on a modest scale, just a few metres across; on the other hand, many true volcanoes fall into the "really quite big" category. |
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You could try prodding Vesuvius with a stick and see if you can get it to start, although that might be unpopular with many in the vicinity. |
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//lasers make everything better// |
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Tell that to both of my eyes that now have plastic lenses
because of lasers. |
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And the precise location of the Werther's limit
usually requires a fudge factor. |
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//about 40TJ.// That's for a hole of 0.5m^2, or 5,000cm^2
I think even a 1cm^2 hole would qualify, taking the energy
cost down by a factor of 5000. That means we're looking at
a 3kW laser running non-stop for a month, and I think I
could handle that (my lab draws about 10kW even when I'm
not doing anything). |
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On the other hand, that's only the energy needed to *melt*
the rock, whereas I need to vaporise it. So, probably
double the energy bill. But it's still manageable. |
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I mean, seriously, why has nobody done this? |
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If one of us thought it up... then odds are it was already tried by somebody fifty-plus years ago and classified. |
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The things taught are So far behind the things actually known it's like really not funny anymore. |
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You would need the earth to be air-tight
too - no doing this through sand. |
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Substances are generally easier to boil
(aka vaporise) at low pressure, but
maybe not so much if you have to supply
all the heat with a laser. |
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//The things taught are So far behind the things actually
known it's like really not funny anymore.// I know - I was
saying the very same thing to my android on the way to Mars
yesterday. |
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That's about where we could be by now. |
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// earth to be air-tight // |
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If the laser's configured to vitrify the sidewall, that won't be a problem. |
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[hippo]'s numbers don't account for beam divergence. I'm not sure what the divergence of such a high power laser would be, but the best laser pointers have a beam divergence of about 0.5mRad, so at 30km the beam diameter would have increased by about 15m. That's a substantial amount more rock to remove, so it would need substantially more power. |
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Condensation of the vaporised rock onto the side
of the borehole would be a continuous issue. |
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//Condensation of the vaporised rock// It would.
Howevertheless, if the rock condenses it will impart energy of
condensation to the walls of the hole, making it easier for the
next laser pulse to re-vaporise the material. |
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I don't think so. Ultimately, it takes X amount of energy to
vaporise Y amount of rock. |
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Thought of something similar earlier: |
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put laser hole driller on planetary explorers, like mars
explorer, to sample geology. I think you could dump
something like a plurality (dozens?) of those cone-weighted
payloads or gombochs that was just a solar panel and a laser
rock drill and some sensors. |
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//that's only the energy needed to *melt* the rock, whereas I need to vaporise it. So, probably double the energy bill. But it's still manageable// - indeed, I couldn't find the data last time I looked, but I've got a better source now (see link). The link says "We are not likely to find a value for latent heat of vaporization of quartz
at its boiling point. The best option is to assume that it is much smaller
than the heat input needed to raise the temperature.". However the per kg energy consumption this analysis comes up with is about 2775kJ/kg, as opposed to the 900kJ/kg I came up with before. So, you'll need about a 50MW continuous laser beam for a month. |
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//I mean, seriously, why has nobody done this?// |
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I think I saw a documentary about this - a team of scientists built a sort of thunderbird four but with lasers on the front, so they could go down inside the Earth's core in and futtle about inside. |
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Did the project leader have a fluffy white cat with a diamond-studded collar, by any chance ? |
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//a documentary about this - a team of scientists built// |
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By "documentary" do you mean "film"? Alternatively, by "built"
do you mean "imagined"? I'm fairly certain that nobody has
actually futtled in the Earth's core, even though futtling would
certainly be useful. |
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"The Core" it's called. You could probably research it on netflix or DVD. |
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With your method, I would predict that lasing from the top would reach a maximum depth where inwelling of semi-molten rock would match beam energy.
At best. Because you're also shooting through the vapour on its way out, which probably incurs a lot of heat transfer to the sides of the hole, leading to further melting and enlargement. If you don't get stymied by water incursion way before then. |
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As you know I favour the strategy involving a nuke and a week's worth of all iron-foundry production. |
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I dunno, [Loris]. It's perfectly possible to drill very high
aspect ratio holes in most materials with a laser. So, the
question is, what changes as the hole gets much, much
deeper? |
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Well, the sides of the hole won't change as the hole gets
deeper, so that's not a problem. However, the vaporised
material has further to go to get out of the hole (and, if it
doesn't get out, it will indeed attenuate the beam). Hence
the cunning use of vacuum. |
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Now, admittedly, when the hole is deep enough, not all the
vapour from one pulse will have escaped the hole before
the next pulse happens so, depending on the pulse rate,
you'll always have a finite amount of rock vapour in the
beam path. Howevertheless, vaporised rock is probably not
going to be as opaque as solid rock. And you could always
increase the interval between pulses as the hole gets
deeper, to give more time for rock vapour to escape. |
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So, yes, you'll lose some energy to absorption along the
beam, but I still say it's worth a shot. |
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At the very least, someone ought to make a pencil-wide
hole through the bottom of Everest as a trial project. |
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On further consideration, we think this idea should be implemented. The technical challenges can be overcome with sufficient research and funding. |
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We consider the following aspects important. |
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1. The project should be carried out in Australia. That way the bore can be named the Koala Super Deep Borehole. |
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2. The hole should be at least 1m in diameter to allow a human explorer to be lowered into it. |
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3. The human needs to be carefully chosen. It must be someone reasonably literate and articulate (to be able to describe their experiences lucidly) but also someone expendable - who has little or no actual value to society and who's loss is insignificant. We suggest that a reporter or presenter from broadcast or print media would be the perfect candidate. |
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Then you could make a movie about the expedition called "Journo to the Centre of the Earth" ... |
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Ooh! I just noticed, there is now a Geology category! I am
pyroclastic with delight. |
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Oh dear, not again. It's the poor sod that has to launder your undergarments that we feel sorry for ... |
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